# Chapter 7 Arrays Arrays Arrays are objects that

• Slides: 55

Chapter 7 Arrays

Arrays • Arrays are objects that help us organize large amounts of information • Chapter 7 focuses on: § § § § array declaration and use bounds checking and capacity arrays that store object references variable length parameter lists multidimensional arrays the Array. List class polygons and polylines mouse events and keyboard events © 2004 Pearson Addison-Wesley. All rights reserved 2

Outline Declaring and Using Arrays of Objects Variable Length Parameter Lists Two-Dimensional Arrays The Array. List Class Polygons and Polylines Mouse Events and Key Events © 2004 Pearson Addison-Wesley. All rights reserved 3

Arrays • An array is an ordered list of values Each value has a numeric index The entire array has a single name 0 scores 1 2 3 4 5 6 7 8 9 79 87 94 82 67 98 87 81 74 91 An array of size N is indexed from zero to N-1 This array holds 10 values that are indexed from 0 to 9 © 2004 Pearson Addison-Wesley. All rights reserved 4

Arrays • A particular value in an array is referenced using the array name followed by the index in brackets • For example, the expression scores[2] refers to the value 94 (the 3 rd value in the array) • That expression represents a place to store a single integer and can be used wherever an integer variable can be used © 2004 Pearson Addison-Wesley. All rights reserved 5

Arrays • For example, an array element can be assigned a value, printed, or used in a calculation: scores[2] = 89; scores[first] = scores[first] + 2; mean = (scores[0] + scores[1])/2; System. out. println ("Top = " + scores[5]); © 2004 Pearson Addison-Wesley. All rights reserved 6

Arrays • The values held in an array are called array elements • An array stores multiple values of the same type – the element type • The element type can be a primitive type or an object reference • Therefore, we can create an array of integers, an array of characters, an array of String objects, an array of Coin objects, etc. • In Java, the array itself is an object that must be instantiated © 2004 Pearson Addison-Wesley. All rights reserved 7

Arrays • Another way to depict the scores array: scores 79 87 94 82 67 98 87 81 74 91 © 2004 Pearson Addison-Wesley. All rights reserved 8

Declaring Arrays • The scores array could be declared as follows: int[] scores = new int[10]; • The type of the variable scores is int[] (an array of integers) • Note that the array type does not specify its size, but each object of that type has a specific size • The reference variable scores is set to a new array object that can hold 10 integers © 2004 Pearson Addison-Wesley. All rights reserved 9

Declaring Arrays • Some other examples of array declarations: float[] prices = new float[500]; boolean[] flags; flags = new boolean[20]; char[] codes = new char[1750]; © 2004 Pearson Addison-Wesley. All rights reserved 10

Using Arrays • The iterator version of the for loop can be used when processing array elements for (int score : scores) System. out. println (score); • This is only appropriate when processing all array elements from top (lowest index) to bottom (highest index) • See Basic. Array. java (page 372) © 2004 Pearson Addison-Wesley. All rights reserved 11

Bounds Checking • Once an array is created, it has a fixed size • An index used in an array reference must specify a valid element • That is, the index value must be in range 0 to N-1 • The Java interpreter throws an Array. Index. Out. Of. Bounds. Exception if an array index is out of bounds • This is called automatic bounds checking © 2004 Pearson Addison-Wesley. All rights reserved 12

Bounds Checking • For example, if the array codes can hold 100 values, it can be indexed using only the numbers 0 to 99 • If the value of count is 100, then the following reference will cause an exception to be thrown: System. out. println (codes[count]); • It’s common to introduce off-by-one errors when using arrays problem for (int index=0; index <= 100; index++) codes[index] = index*50 + epsilon; © 2004 Pearson Addison-Wesley. All rights reserved 13

Bounds Checking • Each array object has a public constant called length that stores the size of the array • It is referenced using the array name: scores. length • Note that length holds the number of elements, not the largest index • See Reverse. Order. java (page 375) • See Letter. Count. java (page 376) © 2004 Pearson Addison-Wesley. All rights reserved 14

Alternate Array Syntax • The brackets of the array type can be associated with the element type or with the name of the array • Therefore the following two declarations are equivalent: float[] prices; float prices[]; • The first format generally is more readable and should be used © 2004 Pearson Addison-Wesley. All rights reserved 15

Initializer Lists • An initializer list can be used to instantiate and fill an array in one step • The values are delimited by braces and separated by commas • Examples: int[] units = {147, 323, 89, 933, 540, 269, 97, 114, 298, 476}; char[] letter. Grades = {'A', 'B', 'C', 'D', ’F'}; © 2004 Pearson Addison-Wesley. All rights reserved 16

Initializer Lists • Note that when an initializer list is used: § the new operator is not used § no size value is specified • The size of the array is determined by the number of items in the initializer list • An initializer list can be used only in the array declaration • See Primes. java (page 381) © 2004 Pearson Addison-Wesley. All rights reserved 17

Arrays as Parameters • An entire array can be passed as a parameter to a method • Like any other object, the reference to the array is passed, making the formal and actual parameters aliases of each other • Therefore, changing an array element within the method changes the original • An individual array element can be passed to a method as well, in which case the type of the formal parameter is the same as the element type © 2004 Pearson Addison-Wesley. All rights reserved 18

Outline Declaring and Using Arrays of Objects Variable Length Parameter Lists Two-Dimensional Arrays The Array. List Class Polygons and Polylines Mouse Events and Key Events © 2004 Pearson Addison-Wesley. All rights reserved 19

Arrays of Objects • The elements of an array can be object references • The following declaration reserves space to store 5 references to String objects String[] words = new String[5]; • It does NOT create the String objects themselves • Initially an array of objects holds null references • Each object stored in an array must be instantiated separately © 2004 Pearson Addison-Wesley. All rights reserved 20

Arrays of Objects • The words array when initially declared: words - • At this point, the following reference would throw a Null. Pointer. Exception: System. out. println (words[0]); © 2004 Pearson Addison-Wesley. All rights reserved 21

Arrays of Objects • After some String objects are created and stored in the array: “friendship” words “loyalty” “honor” - © 2004 Pearson Addison-Wesley. All rights reserved 22

Arrays of Objects • Keep in mind that String objects can be created using literals • The following declaration creates an array object called verbs and fills it with four String objects created using string literals String[] verbs = {"play", "work", "eat", "sleep"}; © 2004 Pearson Addison-Wesley. All rights reserved 23

Arrays of Objects • The following example creates an array of Grade objects, each with a string representation and a numeric lower bound • See Grade. Range. java (page 384) • See Grade. java (page 385) • Now let's look at an example that manages a collection of CD objects • See Tunes. java (page 387) • See CDCollection. java (page 388) • See CD. java (page 391) © 2004 Pearson Addison-Wesley. All rights reserved 24

Arrays of Objects • A UML diagram for the Tunes program: Tunes CDCollection - collection : CD[] - count : int - total. Cost : double + main (args : String[]) : void + add. CD (title : String, artist : String, cost : double, tracks : int) : void + to. String() : String - increase. Size() : void CD - title : String artist : String cost : double tracks : int 1 * + to. String() : String © 2004 Pearson Addison-Wesley. All rights reserved 25

Command-Line Arguments • The signature of the main method indicates that it takes an array of String objects as a parameter • These values come from command-line arguments that are provided when the interpreter is invoked • For example, the following invocation of the interpreter passes three String objects into main: > java State. Eval pennsylvania texas arizona • These strings are stored at indexes 0 -2 of the array parameter of the main method • See Name. Tag. java (page 393) © 2004 Pearson Addison-Wesley. All rights reserved 26

Outline Declaring and Using Arrays of Objects Variable Length Parameter Lists Two-Dimensional Arrays The Array. List Class Polygons and Polylines Mouse Events and Key Events © 2004 Pearson Addison-Wesley. All rights reserved 27

Variable Length Parameter Lists • Suppose we wanted to create a method that processed a different amount of data from one invocation to the next • For example, let's define a method called average that returns the average of a set of integer parameters // one call to average three values mean 1 = average (42, 69, 37); // another call to average seven values mean 2 = average (35, 43, 93, 23, 40, 21, 75); © 2004 Pearson Addison-Wesley. All rights reserved 28

Variable Length Parameter Lists • We could define overloaded versions of the average method § Downside: we'd need a separate version of the method for each parameter count • We could define the method to accept an array of integers § Downside: we'd have to create the array and store the integers prior to calling the method each time • Instead, Java provides a convenient way to create variable length parameter lists © 2004 Pearson Addison-Wesley. All rights reserved 29

Variable Length Parameter Lists • Using special syntax in the formal parameter list, we can define a method to accept any number of parameters of the same type • For each call, the parameters are automatically put into an array for easy processing in the method Indicates a variable length parameter list public double average (int. . . list) { // whatever } element array type name © 2004 Pearson Addison-Wesley. All rights reserved 30

Variable Length Parameter Lists public double average (int. . . list) { double result = 0. 0; if (list. length != 0) { int sum = 0; for (int num : list) sum += num; result = (double)num / list. length; } return result; } © 2004 Pearson Addison-Wesley. All rights reserved 31

Variable Length Parameter Lists • A method that accepts a variable number of parameters can also accept other parameters • The following method accepts an int, a String object, and a variable number of double values into an array called nums public void test (int count, String name, double. . . nums) { // whatever } © 2004 Pearson Addison-Wesley. All rights reserved 33

Variable Length Parameter Lists • The varying number of parameters must come last in the formal arguments • A single method cannot accept two sets of varying parameters • Constructors can also be set up to accept a variable number of parameters • See Variable. Parameters. java (page 396) • See Family. java (page 397) © 2004 Pearson Addison-Wesley. All rights reserved 34

Outline Declaring and Using Arrays of Objects Variable Length Parameter Lists Two-Dimensional Arrays The Array. List Class Polygons and Polylines Mouse Events and Key Events © 2004 Pearson Addison-Wesley. All rights reserved 35

Two-Dimensional Arrays • A one-dimensional array stores a list of elements • A two-dimensional array can be thought of as a table of elements, with rows and columns one dimension two dimensions © 2004 Pearson Addison-Wesley. All rights reserved 36

Two-Dimensional Arrays • To be precise, in Java a two-dimensional array is an array of arrays • A two-dimensional array is declared by specifying the size of each dimension separately: int[][] scores = new int[12][50]; • A array element is referenced using two index values: value = scores[3][6] • The array stored in one row can be specified using one index © 2004 Pearson Addison-Wesley. All rights reserved 37

Two-Dimensional Arrays Expression table Type int[][] Description table[5] int[] array of integers table[5][12] integer 2 D array of integers, or array of integer arrays • See Two. DArray. java (page 399) • See Soda. Survey. java (page 400) © 2004 Pearson Addison-Wesley. All rights reserved 38

Multidimensional Arrays • An array can have many dimensions – if it has more than one dimension, it is called a multidimensional array • Each dimension subdivides the previous one into the specified number of elements • Each dimension has its own length constant • Because each dimension is an array of array references, the arrays within one dimension can be of different lengths § these are sometimes called ragged arrays © 2004 Pearson Addison-Wesley. All rights reserved 39

Outline Declaring and Using Arrays of Objects Variable Length Parameter Lists Two-Dimensional Arrays The Array. List Class Polygons and Polylines Mouse Events and Key Events © 2004 Pearson Addison-Wesley. All rights reserved 40

The Array. List Class • The Array. List class is part of the java. util package • Like an array, it can store a list of values and reference each one using a numeric index • However, you cannot use the bracket syntax with an Array. List object • Furthermore, an Array. List object grows and shrinks as needed, adjusting its capacity as necessary © 2004 Pearson Addison-Wesley. All rights reserved 41

The Array. List Class • Elements can be inserted or removed with a single method invocation • When an element is inserted, the other elements "move aside" to make room • Likewise, when an element is removed, the list "collapses" to close the gap • The indexes of the elements adjust accordingly © 2004 Pearson Addison-Wesley. All rights reserved 42

The Array. List Class • An Array. List stores references to the Object class, which allows it to store any kind of object • See Beatles. java (page 405) • We can also define an Array. List object to accept a particular type of object • The following declaration creates an Array. List object that only stores Family objects Array. List<Family> reunion = new Array. List<Family> • This is an example of generics, which are discussed further in Chapter 12 © 2004 Pearson Addison-Wesley. All rights reserved 43

Array. List Efficiency • The Array. List class is implemented using an underlying array • The array is manipulated so that indexes remain continuous as elements are added or removed • If elements are added to and removed from the end of the list, this processing is fairly efficient • But as elements are inserted and removed from the front or middle of the list, the remaining elements are shifted © 2004 Pearson Addison-Wesley. All rights reserved 44

Outline Declaring and Using Arrays of Objects Variable Length Parameter Lists Two-Dimensional Arrays The Array. List Class Polygons and Polylines Mouse Events and Key Events © 2004 Pearson Addison-Wesley. All rights reserved 45

Polygons and Polylines • Arrays can be helpful in graphics processing • For example, they can be used to store a list of coordinates • A polygon is a multisided, closed shape • A polyline is similar to a polygon except that its endpoints do not meet, and it cannot be filled • See Rocket. java (page 409) • See Rocket. Panel. java (page 410) © 2004 Pearson Addison-Wesley. All rights reserved 46

The Polygon Class • The Polygon class can also be used to define and draw a polygon • It is part of the java. awt pacakage • Versions of the overloaded draw. Polygon and fill. Polygon methods take a single Polygon object as a parameter instead of arrays of coordinates • A Polygon object encapsulates the coordinates of the polygon © 2004 Pearson Addison-Wesley. All rights reserved 47

Outline Declaring and Using Arrays of Objects Variable Length Parameter Lists Two-Dimensional Arrays The Array. List Class Polygons and Polylines Mouse Events and Key Events © 2004 Pearson Addison-Wesley. All rights reserved 48

Mouse Events • Events related to the mouse are separated into mouse events and mouse motion events • Mouse Events: mouse pressed the mouse button is pressed down mouse released the mouse button is released mouse clicked the mouse button is pressed down and released without moving the mouse in between mouse entered the mouse pointer is moved onto (over) a component mouse exited the mouse pointer is moved off of a component © 2004 Pearson Addison-Wesley. All rights reserved 49

Mouse Events • Mouse Motion Events: mouse moved the mouse is moved mouse dragged the mouse is moved while the mouse button is pressed down • Listeners for mouse events are created using the Mouse. Listener and Mouse. Motion. Listener interfaces • A Mouse. Event object is passed to the appropriate method when a mouse event occurs © 2004 Pearson Addison-Wesley. All rights reserved 50

Mouse Events • For a given program, we may only care about one or two mouse events • To satisfy the implementation of a listener interface, empty methods must be provided for unused events • See Dots. java (page 413) • See Dots. Panel. java (page 414) © 2004 Pearson Addison-Wesley. All rights reserved 51

Mouse Events • Rubberbanding is the visual effect in which a shape is "stretched" as it is drawn using the mouse • The following example continually redraws a line as the mouse is dragged • See Rubber. Lines. java (page 417) • See Rubber. Lines. Panel. java (page 418) © 2004 Pearson Addison-Wesley. All rights reserved 52

Key Events • A key event is generated when the user types on the keyboard key pressed a key on the keyboard is pressed down key released a key on the keyboard is released key typed a key on the keyboard is pressed down and released • Listeners for key events are created by implementing the Key. Listener interface • A Key. Event object is passed to the appropriate method when a key event occurs © 2004 Pearson Addison-Wesley. All rights reserved 53

Key Events • The component that generates a key event is the one that has the current keyboard focus • Constants in the Key. Event class can be used to determine which key was pressed • The following example "moves" an image of an arrow as the user types the keyboard arrow keys • See Direction. java (page 421) • See Direction. Panel. java (page 422) © 2004 Pearson Addison-Wesley. All rights reserved 54

Summary • Chapter 7 has focused on: § § § § array declaration and use bounds checking and capacity arrays that store object references variable length parameter lists multidimensional arrays the Array. List class polygons and polylines mouse events and keyboard events © 2004 Pearson Addison-Wesley. All rights reserved 55